The present invention relates broadly to vehicular drive systems for applying power generated in a vehicular engine or motor to the ground for vehicle propulsion. More particularly, the present invention relates to a drive axle assembly for vehicular use, particularly with electric vehicles, which provides differential operation while being externally driven.
Many automobiles are considered "rear-wheel drive" in that the rear wheels propel the vehicle along a roadway. Power is typically transmitted to the rear wheels through a differential mechanism which allows the driving wheels to rotate at independent velocities. This is a requirement for smooth operation during directional changes which creates an inside wheel or tire and an outside wheel or tire. While it is understood that tires provide the frictional contact for vehicle propulsion, in the interest of clarity and simplicity, reference will be made to wheels in the present application which should be understood to mean the wheel/tire assembly unless otherwise designated. When rounding a corner the outside wheel must cover a greater distance than the inside wheel and, if the wheels are not allowed to independently rotate at different velocities, the result is tire scrub resulting in increased wear on the tires and other drive train components, and less than ideal handling.
Typically, gasoline powered vehicles use a mechanical geared transmission connected to a rotatable drive shaft which are used for power transmission from the motor or engine to the wheels. In a rear-drive vehicle, the engine crank shaft typically rotates about an axis along the longitudinal direction of the vehicle, that is extending in the direction of travel. Likewise, the drive shaft rotates in a similar axis which may be vertically offset from the crank shaft. On the other hand, the wheels are intended to rotate about an axis transverse to the direction of travel.
Accordingly, the power must be transferred from rotation about a longitudinal axis to rotation about a transverse axis. The so-called "rear end" of the vehicle accomplishes this. Typically, the wheels are supported on axles which extend inwardly toward a centerline of the vehicle and intersect at the termination of the drive shaft. It would be possible to use bevel gears to transfer the power 90.degree. for application to the wheels. However, mere beveled gears would result in the aforesaid tire-scrub problems. Therefore, automotive manufacturers have developed a differential system having a spider gear assembly. Such an assembly typically uses a ring gear, or crown gear, which is driven by the drive shaft with the ring gear being attached to the spider gear assembly. The spider gear assembly, which will be explained in greater detail hereinafter, creates the differential effect and allows both wheel supporting axles to be driven at the same speed when the vehicle is moving in a straight line, but allows the axle opposite the ring gear to turn slower or faster as the vehicle turns.
In electric vehicle applications, power is available from the electric motor's armature which is typically oriented transversely to the direction of vehicle travel. Further, electric motors do not have the torque available from gasoline motors, yet are capable of producing significantly more revolutions per unit time than gasoline motors. Accordingly, electric motors used in vehicles have significantly different power producing characteristics than do gasoline motors. This is evident in the way electric vehicles apply power to their respective drive trains. Often, the vehicle will have an electric motor driving each wheel independently. This allows each wheel to rotate at its own velocity regardless of the direction of travel of each wheel. Further, electric motors have inherent magnetic coupling which can absorb small variations in speed between different motors at different wheels.
In some applications, it may become desirable to mount a large electric motor in the rear portion of a vehicle with a belt and pulley system for power transmission. Attachment of a belt and pulley system to a solid axle would not allow the wheels of the vehicle to experience the differential effect and would result in the aforesaid tire scrub. Therefore, it is desirable to provide an apparatus which would allow direct axle drive of an electric vehicle by the electric motor while still providing the ability to turn corners with the drive wheels operating at independent velocities.